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vni                       PROTECTIVE COLOUR OF SLOTH                      171

as hooks to keep them suspended from the lower side of a
branch. The three-toed sloth, Uradypus (or "Ai"), has the
exceptional number of nine cervical vertebrae ; the two-toed sloth,
Ckoloepus hojfmanni (or "Unau"), has the equally exceptional
number of six. The hair is long and shaggy, and gets an
adventitious green colour from the presence of minute algae.1 This
gives to the animal the appearance of a lichen-covered bough, a
resemblance which is increased in one species by an oval mark
upon the back, which suggests forcibly a broken end of such a
branch. The likeness of a Sloth to its surroundings is pointed

FIG. 97.—Skull of Three-toed Sloth. Bradyptts tndacti/ftis. Lateral view, fr, Frontal ;
j-n, jugal ; Icr, lachrymal j -max, maxilla ; <najt9 nasal ; pm\ parietal ; s.oc, supra-
occipital ; ty, tympanic. (From Parker and Haswell's Zoology,)
out by Z)r. Siernann,2 who observed that a species occurring in
Nicaragua " has almost exactly the same greyish-green colour as
Tillandsia usncoides, the so-called 'Vegetable Horsehair* common
in the district. . . . If it could be shown that it frequented trees
covered with that plant . . . there would be a curious case of
mimicry between the sloth's hair and the Tillandsia, and a good
reason why so few of these Sloths are seen." The stomach in the
Sloths is complicated in structure, with several chambers ; one of
these gives off a long crescent-shaped caecum- The skull of the
Sloths agrees in a number of particulars with that of the Anteaters.
3 Tlio colour fad us in captivity owing to the disappearance of tlie algae.
a Iu a letter addressed to Dr. Gray, quoted l>y the la-ttm* in a revision of the
Sloths, /Voc. ZooL Soc. 1871, p. 428.